This invention relates to a method of producing freeze-vacuum-dehydrated particulate foods, such as vegetables and meats without resorting to compaction, such that the dehydrated foods will have bulk densities which nearly approach the bulk densities of compacted foods. These foods readily rehydrate to substantially the same distinct particulate states in which they existed prior to freeze dehydration.
Freeze drying of foods is a well-known method for preserving foods so that they can be stored at room temperature or even higher temperatures for long periods of time, provided they are maintained at very low moisture levels during such storage. When properly prepared and stored, such foods may be rehydrated at the time it is desired to consume them, the reconstituted foods being of quality approaching that of freshly prepared foods of the same types.
In recent years, the military forces have found it to be highly desirable to utilize freeze dried foods in compact form so that the foods will occupy appreciably less space in submarines, in aircraft and also when carried on the person. Compact foods are generally formed into cylindrical or rectangular shaped bars which are of a convenient form to be stored and carried.
Various techniques have been used to reduce the volumes and increase the densities of dehydrated foods without destroying the ability to return to substantially the conditions they were in prior to dehydration. The technology for forming freeze dried food bars by compression is complex and costly, requiring considerable handling of the product with a commensurate loss in quality. The techniques heretofore employed required two freeze drying steps and a plasticization step.
One of the methods of producing compacted freeze-dried foods is in accordance with the teachings of Ishler et al, U.S. Pat. No. 3,385,715, whereby a food is first freeze-dried to a low moisture level, such as about 1.1% to about 2.2%. Then the freeze dried food is sprayed with enough water to raise its average moisture content to from about 5% to about 13% so that, when compaction of the freeze-dried food is carried out, the food will not shatter, but will flow while maintaining its cellular structure and the particles of food will adhere to each other when compacted. The added moisture serves as a plasticizer which prevents pulverization of the dried food during mechanical compaction. In order for plasticization to have the desired effect, the food particles must be sprayed with water or an aqueous solution of a gum or the like and then to be permitted to stand long enough for the water to substantially equilibrate throughout the food particles. This equilibration process may take several hours to permit uniform distribution of the added water. After compaction is carried out, the compacted food is usually freeze-vacuum dehydrated to a moisture level sufficiently low to maintain good quality upon storage. This, of course, means that a substantial amount of freeze drying, an expensive process in itself, has to be repeated to remove the water added during the plasticization step. Futhermore, the slowness of the equilibration makes it virtually necessary for the dehydration and compaction process to be carried out in stages rather than in a continuous manner, as would be desirable, resulting in higher production costs and lower product quality.
Various approaches have been made to avoid the above-mentioned repetition of freeze-drying in the production of nutritionally dense food bars. One such approach involves stopping the initial freeze-drying of the food particles to a moisture level of from 5 to about 13% and then compacting the partially freeze-dried food. However, as is well known in the food freeze drying art, the cores of the food particles being freeze vacuum dehydrated remain frozen until most of the moisture that is removed from the food has sublimed and the water vapor formed thereby has migrated through the outer portions of the food particles under the influence of the vacuum. Thus, if the freeze-vacuum-dehydration is stopped at 5-13 percent moisture content as disclosed in the Ishler et al patent, the outer portions of the particles of food are extremely dry while the cores contain essentially their original concentrations of water as ice. If such food particles are then compacted without permitting equilibration to occur, the extremely dry outer portions of the particles shatter and the compacted product has very poor properties. In such a case, it is impossible to restore the original particle sizes and shapes upon rehydration. One solution to this problem is to hold the particles of partially freeze-dried food in closed containers at temperatures have 0.degree. C. to permit equilibration of the moisture from the frozen cores throughout the particles of food. Such equilibration, however, proceeds very slowly, and the larger the food particles, the more slowly equilibration occurs.
U.S. Pat. No. 4,096,283 disclosed a method of speeding up the equilibration process described above through the use of microwave energy to distribute the moisture through the food. However, this process retains the disadvantages inherent in two freeze vacuum dehydration steps with the additional handling of the food required. In U.S. Pat. No. 3,950,560 the disadvantages of two freeze drying steps were avoided by predrying (without freezing) a vegetable in morsel form to a moisture content from about 7 percent to about 18 percent by weight, compressing the predried, vegetable morsels to form a compacted, vegetable mass and then redrying the compacted vegetable mass to a moisture content of less that 5 percent by weight. Prior efforts to produce a satisfactory nutritionally-dense, freeze-dehydrated food product share several undesirable characteristics. Compressed food bars rehydrate slowly. Frequently, warm or hot water is required to rehydrate within a reasonable period of time. Furthermore, those processes which require multiple applications of freeze-vacuum-dehydration, as opposed to a single application, cause the food to be exposed to air for longer periods of time with an increase in deterioration due to oxidative rancidity.
It is an object of this invention to provide an improved method of producing nutritionally dense freeze dried food bars so that upon rehydration the foods will be restored to substantially the same distinct particulate form in which they existed prior to dehydration of the foods and so that the reconstituted foods will be of high quality and as closely comparable as possible to the original raw or cooked foods from which the freeze-dried particulate foods are prepared.
Other objects and advantages will become apparent from the following description of the invention.